1. Field of the Invention
The present invention relates to an apparatus and a method for improving the scan accuracy and quality for flatbed scanners, and more particularly, to an apparatus and a method for improving the scan accuracy and quality by using step motors with a closed loop control technology.
2. Description of the Prior Art
Science progresses have significantly brought many scanners to be manufactured for achieving kinds of requirements. All the reflective documents (such as papers and business cards) and transparent documents (such as projection slides or lantern slides) can be scanned by applying various scanners, and their scanned images can be stored for further processing. Typically, the scanners can be classified into flatbed scanners that put all scanned documents on the top of their transparent windows just like copy machines do, and feed-in scanners feed the scanned documents into the scanners for generating the scanned images. Recently, the scanners are manufactured smaller than before for achieving the purpose of building the scanners into computer cases. Although the build-in scanners have smaller sizes than before, however, the build-in scanners have becoming standard equipment for personal computers.
In addition to the aforementioned requirement for building in the computer cases, the scanners are also required to have a higher resolution for generating scanned images that are more pleasant to the viewer. For example, the resolution is increasing from 300 dpi (dot-per-inch) to 600 dpi or higher today. Conventionally, step motors are usually employed in the scanners for driving image sensors because an open loop control that controls the moving steps of the image sensors can be easily achieved. For example, the step motors can drive a CIS (Contact Image Sensor) to move step-by-step for scanning according to the sizes of the scanned documents.
Usually, the qualities of the scanned images will be significantly influenced by the step motors because the step motors generate resonant frequencies naturally when they work at low rotation speed (usually 150-250 pps (pulses per second)). Moreover, the step motors usually generate noise caused by the step-by-step motions. In addition, the open loop control is impossible to control the actual moving steps of the image sensors because there is no feedback information for confirming the actual moving steps. Another technology for controlling the moving steps is to apply a closed loop control that feeds back the current address of the image sensor for adjusting by using an additional address sensor. An encoder is thus needed to receive and encode the current address obtained from the address sensor to properly move the image sensor. Unfortunately, additional circuits are also required for the additional encoder, which will also significantly increase the cost of the scanner. A need has been arisen to disclose an apparatus, in which all the aforementioned disadvantages can be overcome for improving the scan accuracy and quality of the flatbed scanners.